Waterproof and vapor-permeable soles made of polymeric material for shoes for sports use have now been known for several years.
A sole of this type is disclosed for example in WO2004/028284.
WO2004/028284 discloses a sole which has a structure that comprises a supporting layer which, at least in one preset macroportion, is made of mesh, felt or other diffusely perforated material.
A membrane made of a material that is impermeable to water and permeable to water vapor is associated in an upward region with the supporting layer at least in the preset macroportion that it covers.
A tread made of polymeric material with at least one through opening at the provided macroportion is joined hermetically to the membrane and to the supporting layer at least at the perimeter of the macroportion.
The presence of the opening defines a large section for the exchange of heat and vapor of the membrane with the outside, and therefore said sole structure is capable of utilizing the vapor permeability of the membrane that is impermeable to water and permeable to water vapor.
Soles of this type are particularly adapted for dissipating large quantities of water vapor that form inside the shoes of individuals who have higher-than-average perspiration.
Excessive sweating of the feet can also occur in the case of extremely hot and humid climates as well as in the case of use of shoes in sports activities.
This sole is provided by assembling, by adhesive bonding, the several layers and elements, which are provided separately: the tread, the membrane, the midsole, et cetera.
The mechanical resistance of the macroportion made of mesh, felt or other diffusely perforated material, arranged below the membrane and acting as a supporting element, is particularly important in the manufacturing of said sole.
Simple adhesive bonding of the tread onto the supporting layer, perimetrically with respect to the macroportion, is in fact unlikely to allow their intimate grip, which is needed to provide the required structural strength to the sole.
A sole provided according to the teaching disclosed in WO2006/010578 provides for the overmolding of the lower layer of the sole, which has a macrohole, onto a support and protection layer made of mesh.
In this manner, the lower layer, which also comprises the tread, is jointly connected to the mesh during its molding by injection of polymeric material in the fluid state, which passes through the mesh, embedding it and thus gripping it intimately.
This molding method provides for the insertion of the mesh-membrane package in a first mold and then the lower layer provided with the tread is co-molded onto said package. The material of the lower layer, injected in the liquid state within the mold, by passing to the meshes of the mesh, adheres to the membrane and, by solidifying, thus generates a seal that is impermeable to water, which can no longer seep between the two elements.
The semi-finished component obtained in this manner is inserted in a second mold, in which the upper layer of the sole is co-molded. In this case also, the material of the upper layer, injected in the liquid state inside the mold, adheres to the membrane, and by solidifying generates a seal that is impermeable to water.
Though having considerable effectiveness of the seal of the membrane, this method suffers some drawbacks.
Due to the molding of the polymeric material in the fluid state of the lower layer, seepages of material in fact occur through the mesh which are due to the difficulty in closing the mold completely.
Since it is not possible to compress the mesh completely against the membrane, the material in the fluid state can in fact pass through the meshes of said mesh.
The shape of the mesh in fact does not allow the mold to close hermetically and avoid these seepages, which compromise the aesthetics of the sole and reduce the vapor-permeable portion thereof provided at the macrohole.
This method therefore leads to a large number of production rejects.
Moreover, the method for co-molding the membrane with the tread constrains the choice of materials to be used as tread, since for example it is not possible to use a tread made of vulcanized rubber and of all the materials that do not convert to the liquid state in order to be injected.
The need is therefore felt to provide soles, that are impermeable to water in the liquid state, and permeable to water vapor and obviate the mentioned drawbacks, being able to evacuate large quantities of water vapor from the inside of the shoe.